Typically, modern wind turbines have rotor blades with adjustable pitch angle. The rotor blades can be rotated about their longitudinal axis by use of a pitch drive disposed in the rotor hub. Typically, the pitch drive is actuated electrically or hydraulically. By adjusting the pitch angles of the rotor blades, the power generation of the wind turbine can be controlled as well as an aerodynamical braking of the rotor can be accomplished. Particularly, the rotor blades generate a braking torque when moved into feather position. Thereby, the rotor blades ensure that the rotor is not further accelerated and, thus, the rotor blades form an aerodynamical brake for the wind turbine.
The aerodynamical brake effect of the rotor blades is also used for braking the rotor in case of emergency, e.g. when a failure of the drive system occurs or when the wind turbine is in an uncontrolled condition. Accordingly, it is very important that the emergency system works absolutely reliable even when the wind turbine is damaged or in an uncontrolled state, e.g. due to lightning. Especially, the pitch drive is required to function even in a power outage condition.
A redundant mechanical fallback system has been proposed in DE 197 20 025. Therein, the pitch drive is actuated by energy stored in springs which are coupled to the rotor blade via the same drive train as the electrical regulation drive. Thus, the structure of the drive train becomes very complicated.
As another solution to the problem of emergency backup, uninterruptible power supplies (UPS) for the pitch drive have been proposed, e.g. in chapter 8.4.6 of “Windturbines” by E. Hau, Springer-Verlag, 2000, pages 236 and 237. However, such an approach may fail when the electronics of the wind turbine is damaged, e.g., by lightning.
A further problem associated with the above prior art pitch drive systems is that failure in the electrical drive system leads to a shutdown of the wind turbine so that a yield loss occurs.
In DE 101 16 011, a pitch drive system is proposed which comprises more than one drive train for each rotor blade. However, each of the drive trains comprises an electric motor with battery backup for emergency. Accordingly, the pitch drive system of DE 101 16 011 is still fault-prone in cases where the electronics of the wind turbine is damaged, e.g., by lightning.